The invention relates to a multi-component material, in particular a two-component material with admixtures, for example in the form of filler particles. The invention relates in particular to an implant material which contains a polymer and/or copolymer as well as filler particles. Such implant materials are used to anchor prosthesis components in bone and to reinforce the bone and additionally as the dowel of bone screws or as implants for anchoring screws, for example in the case of composite osteosyntheses. In particular polymer acrylate and especially polymethyl acrylate are used as the polymer. The invention will be described in the following with such an implant material but it also comprises any multi-component material consisting of at least one polymer component and at least one monomeric component as well as admixtures, in particular particle-shaped admixtures.
The above implant materials are known as bone cements and have been clinically tested. They are not absorbed by the body but are biologically inert and are integrated into the osseous system. Known bone cements consist of a polymer component, which is generally in the form of spherules and is also referred to as a bead polymer, a monomeric component and optionally a polymerization catalyst, a stabilizer and an activator. Before they are used, the components are mixed as homogeneously as possible. Bone cements on an acrylate base have proven to be advantageous anchorage materials for prosthesis components. However, the disadvantage of such bone cements consists in the fact that despite improved mixing and application methods the material strength and reproducibility has not yet been able to be optimized reliably. The bone cements can wear in the course of their clinical use and show signs of increasing breakage, as a result of which bone absorption and the consequently inevitable displacement of the implant may arise.
As a rule, filler particles are added to the cited implant materials, in particularly to create a certain porosity which should lead to an improved bond between the implant and the bone as a result of the regrowth of osseous tissue.
It is known from DE-A-29 05 878 that one of the problems with bone cements comprising filler particles, so-called filler cements, consists in the fact that the pores of the filler particles absorb the monomer and that this monomer is thus no longer available for the embedding of the polymer powder. This can result in an incomplete polymerization and in the penetration of the monomer into the circulatory system of the patient. Furthermore, the pores cannot be designed in the desired manner and the strength of the bone cement can possibly be reduced. In DE-A-29 05 878 an attempt is made to solve this problem by temporarily closing the pore volume in order to prevent the penetration of monomers. However, the filler particles should be as freely accessible to the bone as possible and cannot be completely embedded into the bone cement since their free surface would otherwise be no longer accessible to the bone.
In addition to the cited filler particles which according to DE-A-29 05 878 preferably consist of absorbable tricalcium phosphate, radiographic contrast agents and antibiotics for example can also be understood as filler particles in the general sense. The term "filler particle" within the scope of the invention should thus also include, for example, radiographic contrast agents and antibiotics. It has already been shown (Lee A. J. C., Ling R. S. M., Vangala, S. S., Arch. Orthop. Traumat. Surg. 92, 1-18, 1978) that radiographic contrast agents and antibiotics also cause a clear decrease in the strength of the bone cement which can be between 4% and 25%. It has likewise also been demonstrated (Klaus Draenert, Forschung und Fortbildung in der Chirurgie des Bewegungsapparats 2, zur Praxis der Zementverankerung, Art und Science, Munchen, page 39, 1988, ISDN 3-923112-11-4) that in particular the radiographic contrast agents show filling defects in the secondary polymerizing matrix of the bone cement.
In a normal packing of bone cement with 40 g polymer powder, up to 6 g radiographic contrast agent, usually in the form of barium sulfate or zirconium dioxide, is added. Bismuthic compounds can also be used. It is also known from DE-A-29 05 878 that hydroxylapatite can give a sufficient radiographic contrast of the bone cement implant if added in appropriate amounts. The radiographic contrast agents fulfill their function as an X-ray-impermeable material in contrast to the porous filler particles in the narrower sense, even if they are embedded in bone cement.
Apart from the decrease in the strength of the bone cement the addition of radiographic contrast agents of the filler also presents the problem that the admixture of this filler and in particular the reproducibility of the homogeneity of the mixture can lead to not inconsiderable problems with the industrial production of the polymer powder. It has been shown that, despite polished techniques with ball mills and various shakers, individual packings of bone cement frequently contain so-called conglomerates of radiographic contrast agents which cause the monomer to be absorbed with the result that the mixing conditions and the viscosity of this bone cement charge can differ completely from that of normal bone cements. It was also established that the radiographic contrast agents, for example the zirconium dioxide, of various producers can differ and that in particular barium sulfate tends to form the so-called conglomerates.